Nanostarfruits for Chemical Sensing?
- Gold nano particles, top, created by the Rice University lab of Eugene Zubarev take on the shape of starfruit, below, in a chemical bath with silver nitrate, ascorbic acid and gold chloride. Photo courtesy Zubarev Lab/Rice University
- Nanostarfruits begin as gold nanowires with pentagonal cross-sections. Rice chemist Eugene Zubarev believes silver ions and bromide combine to form an insoluble salt that retards particle growth along the pentagons' flat surfaces. Photo courtesy Zubarev Lab/Rice University
Starfruit-shaped gold particles on the scale of nanometers and with tasty implications for medical imaging and chemical sensing have been prepared by scientists from Rice University. These gold nanorods synthesized by Eugene Zubarev and Leonid Vigderman at Rice's BioScience Research Collaborative, could nourish applications that rely on surface-enhanced Raman spectroscopy (SERS).
Enhanced Raman Effect
The researchers found their particles returned signals 25 times stronger than similar nanorods with smooth surfaces. That may ultimately make it possible to detect very small amounts of such organic molecules as DNA and biomarkers, found in bodily fluids, for particular diseases.
There is a great deal of interest in sensing applications. SERS takes advantage of the ability of gold to enhance electromagnetic fields locally. Fields will concentrate at specific defects, like the sharp edges of these nanostarfruits, which could help detect the presence of organic molecules at very low concentration.
SERS can detect organic molecules by themselves, but the presence of a gold surface enhances the effect by several orders of magnitude, Zubarev said. Moreover, the use of these nanostarfruits, which further boost that stronger signal by a factor of 25 is significant.
The scientists grew batches of the star-shaped rods in a chemical bath. They started with seed particles of highly purified gold nanorods with pentagonal cross-sections developed by Zubarev's lab in 2008 and added them to a mixture of silver nitrate, ascorbic acid and gold chloride.
Over 24 hours, the particles plumped up to 550 nanometers long and 55 nanometers wide, many with pointy ends.
The particles take on ridges along their lengths; photographed tip-down with an electron microscope, they look like stacks of star-shaped pillows.
Why the pentagons turn into stars is still a bit of a mystery. When Zubarev and Vigderman added a common surfactant, cetyltrimethylammonium bromide (aka CTAB), to the mix, the bromide combined with the silver ions to produce an insoluble salt. The salt is believed to form a thin film on the side faces of rods which partially blocks them. This in turn slowed down the deposition of gold on those flat surfaces and allowed the nanorods to gather more gold at the pentagon's points, where they grew into the ridges that gave the rods their star-like cross-section. The researchers tried replacing silver with other metal ions such as copper, mercury, iron and nickel. All produced relatively smooth nanorods.
The researchers also grew longer nanowires that, along with their optical advantages, may have unique electronic properties. Ongoing experiments with Stephan Link, an assistant professor of chemistry and chemical and biomolecular engineering, will help characterize the starfruit nanowires' ability to transmit a plasmonic signal. That could be useful for waveguides and other optoelectronic devices.
But the primary area of interest in Zubarev's lab is biological. If the surface roughness can be modified such that biological molecules of interest will adsorb selectively on the surface of the rugged nanorods, then one can start looking at very low concentrations of DNA or cancer biomarkers. There are many cancers where the diagnostics depend on the lowest concentration of the biomarker that can be detected.
The National Science Foundation and Welch Foundation supported the research.
Vigderman L and Zubarev R.: Starfruit-Shaped Gold Nanorods and Nanowires: Synthesis and SERS Characterization, Langmuir (2012), DOI:10.1021/la300218z